Short answer: cut square, deburr, run the flare nut on first, then flare in stages with the right die for your fitting standard. Skipping any one of those four steps is where most home-built brake lines go wrong.
Know your flare standard before you start
Not all flares are the same shape, and a die for one standard will not seal correctly against a fitting built for another:
- SAE 45° double flare, the traditional standard on most US-market vehicles. The tube end is folded back on itself twice, which is what makes it a “double” flare, giving a thicker, more durable sealing surface at the cost of an extra step.
- ISO/DIN bubble flare, common on many European and newer vehicles. A single rounded “bubble” shape rather than a folded double flare, and not interchangeable with SAE fittings even though the tube sizes can match.
- AN 37° flare, the racing/performance standard, a single flare at a different angle than SAE, used with AN-style fittings.
Using the wrong flare on the right-size tube is one of the most common causes of a brake line that looks fine and still leaks or fails a bleed, the flare seats against the fitting at the wrong angle or depth even though the threads go together.
Tube size and the thread it mates to
The tube size tells you which die to clamp into the tool, and on an SAE 45° flare it also tells you the thread you’re chasing on the fitting side. 3/16″ tube mates to a 3/8-24 flare nut, 1/4″ tube to 7/16-24, 5/16″ to 1/2-20, and 3/8″ to 9/16-18. Get the wrong tube size in the die and the flare comes out the right shape but the wrong diameter, so the nut either won’t start or spins loose past the seat.
Watch for AN fittings here too. AN’s 37° flare shares the exact same thread as SAE 45° at three of those four sizes, 3/16″, 5/16″ and 3/8″ all thread on cleanly whether the flare underneath is 37° or 45°. A nut tightening down smoothly tells you nothing about whether the angle matches. If you’re not sure which standard a fitting expects, check the flare it’s already sealed against rather than trusting the thread alone. (More on that overlap and where it bites people.)
Step by step
- Cut the tube square. A tubing cutter, not a hacksaw, an angled cut won’t flare evenly and won’t seal.
- Deburr inside and out. Any burr left on the inside edge will show up as a stress point in the finished flare, and on the outside it’ll keep the flare nut from seating flush.
- Slide the flare nut onto the tube before you flare it. This is the step everyone forgets once, exactly once. Once the flare is formed, the nut can’t get past it.
- Clamp the tube in the die at the correct depth for your tool and flare standard, too much or too little stickout changes the finished flare’s diameter and thickness.
- Form the flare in stages rather than cranking it down in one motion, backing off periodically to check the shape. Rushing this step is the most common cause of a cracked or uneven flare.
Why a correctly-cut flare still cracks or leaks
A flare that starts right can still fail for reasons that have nothing to do with the cut or the deburring. Too much stickout past the die spreads the flare out thin, and it splits under pressure instead of holding. Too little stickout undersizes the cone so it never makes full contact with the seat, and it can look tightened down while it still weeps. The tube itself can shift in the die if the clamp isn’t snug before the punch comes down, which throws the flare off-center even when every earlier step was done right.
Torque causes its own failures, in both directions. Too loose leaks right away. Too tight can crack the flare or gall the seat, especially on the softer brass fittings common on aftermarket hardware. As a general guide for SAE double-flare fittings, 3/16″ tube runs about 10 to 15 ft-lb, 1/4″ tube 12 to 18 ft-lb, 5/16″ tube 20 to 25 ft-lb, and 3/8″ tube 25 to 35 ft-lb. Your vehicle’s own service manual spec overrides any of this if the two disagree, and a fitting that’s already sealing doesn’t need another quarter turn to feel more finished.
Picking the right kit
All three BrakeQuip flaring kits look similar in the box, a frame, a set of dies, a cam or screw mechanism, so it’s easy to grab the wrong one buying on price alone. BQ351 is SAE 45° double flare only, built to also handle stainless, which not every 45°-only tool can do since stainless work-hardens fast and tears a softer die. BQ352 skips SAE entirely and covers AN 37° and DIN, for cars already running braided AN lines or European bubble-flare fittings. BQ350 is the one to reach for if you don’t know yet which standard the job needs, or if it might need more than one, it covers SAE, AN, and DIN with interchangeable turrets in a single tool.
One flare, tested before it’s trusted
Whatever tool you use, pressure-test the finished line before it goes back into service, a flare that looks perfect can still leak under the pressure spikes a brake system actually sees, which a visual check alone won’t catch.